This invention relates to a method and apparatus for removing wall ties, to a procedure involving the method and the apparatus for replacing wall ties and to wall ties for use in the procedure.
In cavity wall construction, wall ties are used to link the inner and outer leaves of masonry at intervals, typically of four or five courses of masonry elements such as bricks. In recent cavity wall construction, such ties have been made of galvanised steel and very recently plastics material such as polypropylene have been proposed for use in wall ties.
However, in early cavity wall construction in the 1920's and 30's, mild steel wall ties having water resistant enamel coatings were used, since galvanised ties were not then available.
During the life of a cavity wall, water penetrating the outer leaf may attack and severely corrode unprotected mild steel wall ties. This may take place for example at defects in the enamel coating. The corroded steel expands and splits into weak flakes, heavily oxidised, and having a much greater thickness than the original ties.
This expansion of the corroded steel ties causes opening of the mortar joints and lifting of the masonry above. Horizontal cracks appear in the outer leaf of masonry along the mortar joints in which the wall ties have been positioned. In severe cases, the lifting of the outer leaf of the wall may cause grave structural defects. Additionally, the loss of strength in the ties can result in the leaves of the cavity wall bulging apart. These effects could cause collapse of the wall.
Previously proposed remedial methods have been unsatisfactory.
For example, it has been proposed to locate the original ties, remove the mortar of the outer leaf of masonry and then, using a bolster chisel, hammer the original tie away from the outer leaf so that it no longer bridges the cavity. The inner and outer leaves must then be reconnected and this is done by drilling through the masonry elements such as brickes, inserting an expansible tie rod and mechanically or frictionally engaging the tie rod with the masonry of the inner and outer leaves.
This method may cause damage to the inner leaf, because the remnants of the original tie may be driven through the inner leaf and the transmission of the hammering forces may damage internal plaster work. Additionally, the insertion of substitute ties through the outer leaf by drilling through the brickwork leaves unsightly traces that tie replacement has been carried out. The expansible ties used are extremely expensive and are not particularly secure because they rely merely on friction to hold them in place.
Another method which has been suggested to resecure the inner and outer leaves together is the use of urethane foam bonding material which bonds to the inner and outer leaves within the cavity and holds them together. While this prevents the masonry leaves from bulging apart, it does not prevent the corrosion and "growth" of the original ties which can still cause severe structural damage to the bulding by forcing apart the courses of masonry in the outer leaf.
A substitute form of tie which has been proposed as an alternative to the friction tie referred to above involves the insertion through drilled holes in the inner and outer leaves of a tube which carries at one end a double capsule of components which will form an epoxy mortar when mixed together. A threaded rod is forced into the tube, the rod having a spade drill formed leading end which penetrates the capsule, releasing the epoxy material, which then forms the mortar around the threaded rod and embedded in the inner leaf. The part of the threaded rod embedded in the outer leaf is then grouted in with an epoxy mortar. The resulting tie is much more secure than the previously described friction tie but is again inserted through drilled holes in the masonry, the operator being unable to see what is happening at the inner leaf. The resulting drilled holes remain visible after remedial work has been carried out.
As referred to above, the more insertion of a new wall tie to secure the inner and outer leaves together only partly cures the problem in that it is still necessary to remove the expanding corroding wall ties from at least the outer leaf of the wall. There may typically be two hundred and fifty wall ties involved in a modest sized house.
A further, drastic remedy is available to cure the problem of wall tie corrosion and this is to demolish the outer leaf of masonry, remove the wall ties manually and rebuild the outer course of masonry, inserting ties in the inner and outer leaves as work progresses. This is extremely costly although very effective.